The present invention relates to detection of cracks, etc., in surfaces, particularly to detection of submicron scale cracks and other surface anomalies, and more particularly to the detection of cracks and other mechanical and chemical surface anomalies using Positron Emission Tomography (PET).
The detection of cracks, voids, and porous regions in metal parts is a generic problem in the field of manufacturing and long-term storage of such parts. This is of particular concern with special significance to the maintenance of the nuclear stockpile. Present radiographic techniques are prone to miss small cracks, and similar features in a general survey, and are difficult to apply in some circumstances. For example, the minimum crack width detectable by computed tomography is about 40 micrometers, or by neutron radiography is about 1 millimeter. Microscopic visual inspection suitable for very small features is limited by its narrow field of view to scanning small areas only. These prior above-identified techniques are among those used routinely for detecting cracks, voids, and fatigue-related failure of mechanical and structural components, ranging from airplane wings to bridge abutments.
PET is a known medical imaging technology. By recognition that PET can be utilized in fields other than medical, the present invention enabled this known medical technology to be applied so as to enable the detection of submicron scale cracks and other mechanical and chemical surface anomalies. Thus the present invention involves the application of PET to detect the presence of cracks and similar mechanical features on surfaces of metallic or mechanical parts, or other materials, and to locate those features with millimeter spatial resolution. It was also recognized that PET technology can be applied to detect very small features in a scanning mode suitable for application to surveillance of the nuclear stockpile, for example, and other engineering applications. In carrying out the method of the present invention, a radioactive gas is directed onto the surface of a part to be inspected, the gas is pumped away leaving a fraction of a monolayer of gas residue on the surface of the port, after which coincident gamma-rays are detected to determine the location and shape of the cracks, voids, porous regions, etc., and calculating the width, depth, and length thereof. Detection of 0.01 xcexcm wide by 10 xcexcm deep cracks is possible with the method of the present invention.
It is an object of the present invention to detect submicron scale cracks and other mechanical and chemical surface anomalies.
A further object of the invention is to detect submicron scale cracks in metallic or other material parts using PET.
A further object of the invention is to provide an application of PET to detect surface regions of differing chemical reactivity.
Another object of the invention is to utilize PET in a scanning or survey mode to simultaneously detect mechanical and chemical features over large interior or exterior surface areas of parts as large as about 50 cm in diameter.
Another object of the invention is to provide a method for detecting cracks and weld integrity on the interior surface of a spherical part using any positron-emitting radioisotopes in PET.
Another object of the invention is to provide a method which utilizes any type of position-sensitive gamma-ray detectors for surface feature detection and characterization by PET.
Another object of the invention is to provide an application of PET to detection of submicron scale cracks and other mechanical and chemical surface anomalies using positron-emitting radioisotopes which can be delivered to the surface via gaseous residue by solution or fluid applied to the surface, by aerosol sprayed on the surface, by high-pressure gas, solution, fluid, or spray on the surface; or by producing positron-emitting radioisotopes on the surface by activation of a pre-existing material (e.g., an oxide layer).
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. Basically, the present invention involves PET applied to detect submicron scale cracks and other mechanical and chemical surface anomalies. The method or technique of this invention has sufficient sensitivity to detect single voids or pits of submillimeter size, and single cracks or fissures of millimeter-scale length, micrometer-scale depth, and nanometer-scale width. The techniques or method of this invention can also be applied to detect surface regions of differing chemical reactivity, as well as in a scanning or survey mode to detect features over large interior and exterior surface areas. Basically, the method or technique involves exposing a surface to short-lived radioactive material for a time period; removing the excess radioactive material to leave a surface residue; determining the location and shape of the cracks, voids, porous regions, etc., due to the xe2x80x9cbrightxe2x80x9d spots caused by increased surface areas of the cracks, etc.; and calculating the width, depth, and length thereof. Cracks as shallow as 10 xcexcm and wide as 0.01 xcexcm are detectable with this method.